Carboranes are useful as burning rate modifiers in solid propellant grains, n-hexyl carborane being particularly useful and preferred because it is a liquid having a melting point of about -50.degree.. Heretofore carboranes have been prepared by reaction of acetylenic compounds with diligand derivatives of decaborane, B.sub.10 H.sub.12 L.sub.2, (L=Lewis base) or solutions of decaborane and a Lewis base which react as the diligand derivatives. Acetonitrile and lower dialkylsulfides are representative of appropriate Lewis bases, and, when forming n-hexyl carborane, the acetylenic compound used in 1-octyne. These prior preparations of alkyl carboranes are characterized by low yields of about 30 percent. Heying et al, A New Series of Organoboranes, I, Carboranes from the Reaction of Decaborane with Acetylenic Compounds. Inorganic Chemistry, Vol. 2, p 1089 (1963); Fein, et al, Carboranes II, The Preparation of 1- and 1,2-Substituted Carboranes, ibid, Vol. 2, p 1115 (1963), Hill, et al, Kenetics and Mechanism of Carborane Formation, ibid, Vol. 14, p 1244 (1975). Hill, et al report (at p 1246) that yields of carboranes were independent of temperature over the range of 38.degree.-100.degree. C for octyne. Temperatures in excess of about 100.degree. C have not been used in prior art reactions because the reactant decaborane derivative solutions decompose or degrade. Low yields are a substantial economic consideration as starting materials are expensive, decaborane, for example, is currently priced in quantity at about $800 per pound, and the product recovery and purification steps are tedious and expensive.